The primary nucleotide sequence of the biologically active proviral genome of simian sarcoma virus (SSV) has been determined in order to understand the molecular mechanisms of transformation by this only known primate retrovirus with oncogenic potential. These studies have revealed that the cellular oncogenic sequences acquired by SSV are activated in the virus-transformed cells by virtue of their regulatory sequences for initiation and termination of transcription and translation provided by the retrovirus. The availability of predicted amino acid sequences for the SSV transforming gene, as deduced from nucleotide sequence analysis, and its comparison to proteins with known amino acid sequences of proteins with known cellular function, made it possible to demonstrate that the SSV transforming gene shares primary structural homology with a potent mitogen, a platelet-derived growth factor (PDGF). These results, thus, constitute the first demonstration of an onc gene product encoded by a retrovirus which has been identified to correspond to a cellular gene with known function. Previous studies from our laboratory have detected SSV onc gene transcripts in specific human malignancies, such as fibrosarcomas, osteosarcomas, and glibolastomas. These findings corroborate the present finding that the SSV onc gene may encode for a protein with mitogenic activity similar to PDGF which exerts its growth-promoting activity on fibroblasts, tumor muscle cells, and glial cells. These results suggest that malignant transformation by the SSV onc gene or its cellular homologues, in certain instances, may involve constitutive expression of a protein which could act as a potent mitogen similar to PDGF.